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Full text of "Alexander von Humboldt"

ALEXANDER VON  HUMBOLBT.                181
mystery, for now every regular or irregular
ment of this force is observed on the most distant
spots of the earth, and instruments as well as the
senses are now so sharpened that Humboldt assures
us the persons in charge of the observatories can at
certain times take down observations every two-and-
half minutes for twenty-four hours, and are able to
measure a decrease of the magnetic power by ^r<r cnnr
degree.
Humboldt calls the north-light (the polar light or
aurora borealis) the end of a magnetic storm, and con-
siders it a phenomenon which, since Faraday has dis-
covered  that  the magnetic  power  developes   light,
stands in  the   most  intimate  relation  with   earth-
magnetism.      In the morning,  the evening appear-
ance   of this discharge of light is pre-indicated   by
irregularity   in  the course of the magnet,  and this
shows that a disturbance must have taken place in the
equable distribution of the magnetic power.    Accord-
ing   to    Humboldt,    the    north-light   is  a   kind   of
magnetic   discharge,   (as   the lightning restores the
disturbed equilibrium of the atmosphere)  which is
sometimes so violent that it has been seen in full day-
light (at liowervorn in 1786).    This phenomenon has
not only been seen at the 3STorth and South Poles,
but Humboldt has ascertained that it has been met
with also in the tropics, even in  Mexico and Peru
towards the north, so that the spectator always has
his own northlight according to the angle of view.
But the   explanation which Humboldt fgives  of the
existence of this north light, which only received its
name from the direction in which it was most fre-
quently observed,   is   of the  greatest  interest.    He
considers it a self-luminousness of the earth,  a light
development of the planet, in a word an earthlight in
opposition to the sunlight.    The planet Venus also
phosphoresces  on that  side not illuminated by the
sun,   and Humboldt thinks   it   very probable that
planets and moons, besides the received and reflected